The present invention relates to systems for projecting images from diffractive phase plates, and particularly to, systems for projecting images encoded with different image information arranged along a path which have phase shifts aligned to reconstruct visible images in the Fraunhofer (or far-field) diffraction region. These systems are useful for displaying monochrome or color animation, and static color images.
A diffractive phase plate (DPP) is a phase object that is capable of reconstructing an image in the Fraunhofer (of far-field) diffraction region of the DPP. Phase information is encoded into the DPP using a spatially varying surface-height or index of refraction distribution. Methods to encode information about the reconstructed image in the phase distribution of the DPP are described in academic literature, such as Stark et al., xe2x80x9cDesign of phase gratings by generalized projections,xe2x80x9d J. Opt. Soc. Am. A, Vol. 8, No. 3, March 1991, pp. 566-571, Duparrxc3xa9et al., xe2x80x9cInvestigation of computer-generated diffractive beam shapers for flattening of a single-modal CO2 laser beams,xe2x80x9d Applied Optics, Vol. 34, No. 14, May 10, 1995, pp. 2489-2497, and Yoshikawa et al., xe2x80x9cPhase optimization of a kinoform by simulated annealing,xe2x80x9d Applied Optics, Vol. 33, No. 5, Feb. 10, 1994, pp. 863-868.
DPPs differ from amplitude-modulated diffractive plates in that they are more efficient, i.e., a greater percentage of the incident light is directed into the reconstructed image, and DPPs may be replicated in high volume and at low cost. Other features of DPPs are that no additional image-forming optics are generally required between the DPP and the reconstructed image, and when the image reconstruction occurs in the Fraunhofer (or far-field) diffraction region, the reconstructed image is in focus at all distances in the diffraction region. Further, lateral magnification of a reconstructed image from a DPP is directly proportional to the distance between the DPP and the plane of observation.
Efficient image reconstructions with DPPs may be accomplished by shaping the surface-relief (or blaze) profile of the DPP, such as the projection onto constrained sets algorithm (POCS), as described Stark et al., supra. Furthermore, there are several methods DPP elements can be used to produce DPP replicas, including optical lithography, electron-beam lithography, and single-point laser pattern generation, examples of fabrication methods are described in: U.S. Pat. No. 4,846,552 by Veldkamp et al, issued Jul. 11, 1989; U.S. Pat. No. 5,148,319 to Gratix et. al. issued Sep. 15, 1992, and academic publications by Gale et al., xe2x80x9cFabrication of continuous-relief micro-optical elements by direct laser writing in photoresists,xe2x80x9d Opt. Eng. 33, pp. 3556-3566 (1994), and Faklis et al., xe2x80x9cContinuous phase diffractive optics using laser pattern generation,xe2x80x9d SPIE Holography Tech. Group Newsletter, Vol. 2, July 1993.
Typically, the use of DPPs in projection systems have been limited to illuminating isolated DPPs with laser light to reconstruct images encoded on each DPP. The reconstructed images produced by such systems provide only static and monochromatic images.
In addition, holographic image projection systems have been proposed to provide two or three dimensional images from sequenced holograms, but have shown limited practicability and only use holograms. Examples of such holographic image projection system are shown in: U.S. Pat. No. 3,632,181 to Lee, issued Jan. 4, 1972; U.S. Pat. No. 3,625,585 to St. John, issued Dec. 12, 1971; U.S. Pat. No. 4,007,481 to St. John, issued Feb. 8, 1977; and U.S. Pat. No. 4,595,252 to Lorimer, issued Jun., 17, 1986.
Such holograms used in these projection systems are distinct from DPPs. A hologram is a recording of the interference pattern between two optical fields, an object beam and a reference beam. The resulting interference pattern is generally recorded using a volume-type recording material, such as a thick layer of photosensitized emulsion. Volume-type recording materials enable one to produce efficient (bright) image reconstructions through the use of Bragg effects within the recording volume. In contrast, in a DPP only object information is encoded as phase information. There is no reference beam, and therefore there is no interference pattern recorded between object and reference beams. Accordingly, a DPP is not a hologram. Further, holograms do not provide the advantageous features of DPPs stated earlier.
It is the principal object of the present invention to provide an improved system for projecting images having a group of diffractive phase plates in which when multiple phase plates are illuminated a composite image is formed by the coincident superposition of reconstructed images in the Fraunhofer (or far-field) diffraction region.
Another object of the present invention is to provide an improved system for projecting images in which monochomatic and color animation is provided by moving a group of multiple diffractive phase plates along a path relative to a light source illuminating the plates.
A further object of the present invention is to provide an improved system for projecting images in which static color images are provided by combining reconstructed images from diffractive phase plates representing different color channels.
Briefly described, the present invention embodies a system for projecting images encoded onto diffractive phase plates (DPPs). The system includes a group of diffractive phase plates each encoded with different image information which have phase shifts aligned to reconstruct visible images in a Fraunhofer (or far-field) diffraction region. The phase plate are arranged along a path. A light source illuminates the plates and provides, when multiple plates from the group are illuminated, a composite image representing image information encoded in the phase plates illuminated. This composite image is a superposition of reconstructed images from the phase plates illuminated in which the reconstructed image are coincident with each other in the composite image.
The light source, preferably a laser, produces a beam to illuminate an area along the path of the plates. Image information is contributed from each phase plate into the composite image in proportion to the overlap of the plates in this area.
The system may further include a mechanism for moving the plates along their path such that images projected from the system in the Fraunhofer (or far-field) diffraction region represent the composite image from successively different illuminated multiple plates or the reconstructed image from a single plate.
Images projected by the system may be monochromatic or color. For color imaging, each diffractive phase plates in the system represents a set of plates of different color channels. For each color channel, the above mentioned light source represents a separate light source for illuminating the phase plates of each color channel to provide a composite image from such plates in each color channel. Optics may be provided for combining the composite images from multiple color channels, or for combining images reconstructed from each plate of a different color channel in a set.
One feature of the present invention is that the frame rate required to produce an animation sequence is reduced substantially compared to that produced by conventional imaging (or movie) systems, where the frame rate must be sufficiently high so has to produce flicker-free animation or imagery. In the case of DPP-generated images, each DPP may represent a different DPP frame and there is a smooth transition from one DPP frame to another irrespective of the frame rate, wherein the reconstructed image evolves continuously from one image to the other in proportion to the area of overlap of the given DPP frames with respect to an illumination beam. Accordingly, the number of frames to produce a repetitive-motion animation sequence is reduced compared to that produced by conventional imaging (or movie) systems.
Another feature of the present invention is that the reconstructed images of the composite images remain in focus and in registration, i.e., coincidence with each other, at any plane in the Fraunhofer diffraction region of the DPPs.
Yet another feature of the present invention is that by reconstructing images from DPPs in the Fraunhofer diffraction region the reconstructed composite image is invariant to the position of the DPPs relative to the illumination of the plates.